Electrochemistry Communications 2 Ž2000. 782–785 www.elsevier.nlrlocaterelecom

Amperometric determination of ascorbic acid at a ferricyanide-doped Tosflex-modified electrode Jyh-Myng Zen ) , Dong-Mung Tsai, Annamalai Senthil Kumar, Venkataraman Dharuman Department of Chemistry, National Chung-Hsing UniÕersity, Taichung 402, Taiwan, ROC Received 14 August 2000; received in revised form 1 September 2000; accepted 5 September 2000

Abstract Tosflex, a perfluoro-anionic exchange membrane, is not studied as much as Nafion for electroanalytical applications. In this study, electrocatalytic oxidation of ascorbic acid ŽAA. was demonstrated using a ferricyanide-doped Tosflex-modified electrode in pH 5 phosphate buffer solution. The modified electrode showed good stability over the studied pH range of 2–12. The electrocatalytic oxidation of AA on the modified electrode follows the surface-saturation kinetics in terms of Michaelis-Menten ŽMM. mechanism. The analytical estimations were performed amperometrically under hydrodynamic conditions at an applied potential of 300 mV versus AgrAgCl. A linear response was observed in the range of 0–50 mM with a regression coefficient of 0.998. q 2000 Published by Elsevier Science S.A. Keywords: Tosflex; FeŽCN. 63yr4y; Ascorbic acid; Mediated oxidation

1. Introduction The development of amperometric sensors for the determination of ascorbic acid ŽAA., i.e., vitamin C, is important because of its crucial role in neurochemistry and industrial applications w1,2x. Several techniques were reported for AA determination including spectroscopic, chromatographic, enzymatic and electroanalytical methods w2– 5x. Compared to other options, electroanalysis has the advantages of simplicity and high sensitivity. Ferricyanide was proved an effective redox mediator for AA oxidation in homogeneous solution phase as well as in polymer matrix w6–10x. We report here the application of the ferricyanide-doped Tosflex-modified electrode ŽFDTME. for the detection of AA. Tosflex, a new class of perfluorinated polycationic polymer, was considered as a suitable alternate for polyŽ4-vinlylpyridine. ŽPVP. w11,12x and rarely studied in electrochemical applications w13,14x. For comparison, the activity of the ferricyanide-doped PVP-modified electrode ŽFDPME. was also investigated. The FDTME was proved to possess an excellent mediated activity for AA over a wide pH range compared to the FDPME. The reaction was found to follow the surface )

Corresponding author. Fax: 886-4-2862547. E-mail address: [email protected] ŽJ.-M. Zen..

saturation kinetics similar to the enzymatic catalytic reaction.

2. Experimental 2.1. Reagents The Tosflex membrane ŽIE-SA 48. was obtained from Tosoh Soda, Japan. Ascorbic acid and PVP were obtained from Aldrich. All the other compounds used in this work were prepared from ACS-certified regent grade chemicals without further purification and dissolved in doubly distilled deionized water. Unless otherwise noted, a 0.1 M, pH 5 phosphate buffer solution ŽPBS. was used for all electrochemical measurements. 2.2. Apparatus Voltammetric measurements were carried out with CHI Model 660 electrochemical workstation ŽCH instruments, Austin, TX, USA.. The three-electrode system consists of one of the flowing working electrode: GCE, FDTME or FDPME, an AgrAgCl reference ŽModel RE-5, BAS. and a platinum auxiliary electrode.

1388-2481r00r$ - see front matter q 2000 Published by Elsevier Science S.A. PII: S 1 3 8 8 - 2 4 8 1 Ž 0 0 . 0 0 1 2 1 - 1

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2.3. Procedure The dissolution of Tosflex membrane was carried out according to the procedure described in the literature w15x. Around 2.5 g of finely cut dry membrane and 10 ml of water y methanoly 2-propanol aqueousy alcoholic solution was heated to the boiling point at atmospheric pressure under reflux and stirring for ; 20–50 h. After cooling, the undissolved membrane was separated by centrifugation and a clear, yellowish solution was collected. The concentration of the dissolved polymer was determined gravimetrically from an evaporated portion of the solution. The optimal coating solution was brought to a final concentration of 1.3 wt% by dilution with methanol. GCE, 3 mm diameter, was first polished with polishing kit ŽBAS., rinsed with de-ionized water then cleaned by ultrasonically in 1:1 nitric acid and deionized water successively. Then 5 ml of Tosflex Žor PVP. containing solution was spin-coated onto GCE at a spin rate of 2500 rpm. A uniform thin film was formed by evaporation of the solvent after 3 min of spinning. For the loading of FeŽCN. 63yr4y couple into Tosflex or PVP, the polymer-coated GCE was either by dipping in 1 mM FeŽCN. 63y for 10 min followed by gentle washing with double distilled water or by cycling in the CV range of 0.1 to 0.5 V until the voltammogram reached saturation. The electrodes were then cycled continuously in the base electrolyte solution at a scan rate of 100 mV sy1 until the voltammograms became stable. Normally it took 3–5 min. The Tosflex coating solution was optimized as 1.3 wt% as studied with FeŽCN. 63y loading in pH 5 PBS. For the amperometric analysis, the solution was stirred at a stir mode of 3 in the instrument.

3. Results and discussion Fig. 1 shows the typical CV response of the FDTME in pH 5 PBS at a scan rate of 100 mV sy1 . The appearance of X a clear redox peak at a formal potential Ž E o . of 200 mV versus AgrAgCl confirms the electrostatic entrapment of FeŽCN. 63yr4y couple in Tosflex membrane ŽFig. 1b.. The characteristics of the reversible redox process can shift from adsorption-controlled to diffusion-controlled as the scan rate increases. At Õ - 50 mV sy1 , the E logŽ i p .r E logŽ Õ . is close to unity and it is equal to 0.5 at faster scan rates. Meanwhile, the D Ep values also increase from ; 30 to 100 mV upon increasing the scan rate. The FDTME shows catalytic oxidation signal in the presence of AA as indicated in Fig. 1c. The oxidation potential of AA decreases by ; 200 mV in overpotential than that at a bare X GCE. It occurs at the E o of FeŽCN. 63yr4y indicating clearly that the reaction is mediated by FeŽCN. 63y w8x.

Fig. 1. CV responses in pH 5 PBS at a scan rate of 100 mV sy1 at Ža. GCE, Žb. the FDTME and Žc. with the addition of 1 mM AA in Žb..

Further evidence of catalytic oxidation is that the cyclic voltammograms turn into ‘S’ shape as the concentration of AA increases w16x. The effect of solution pH on Epa , i pa and D Ep at the FDTME in the absence and presence of AA was studied and the results obtained are shown in Fig. 2. The D Ep was found to slightly increase up to pH 9 and quickly increase when pH ) 9. This indicates that the diffusion-controlled process is dominate after pH ) 9. The FDTME shows an extraordinary stability in wide pH ranges as indicated in Fig. 2B. Ultimately, the FDTME is ideal and useful for analytical detection of biological compounds under physiological pH. The FDTME shows a maximum response for AA oxidation at pH 5 as shown in Fig. 2C. Considering the p K a s 4.10 for AA, it is the anionic form of AA involved in the effective oxidation on the FDTME w17x. The mechanistic aspect of the system was further studied. The fact that D Ep in acidic pH’s are smaller than 60 mV indicating the thin layer behavior of FeŽCN. 63y at this electrode. The surface excess Ž Gs . was 9.02 =10y1 1 mol cmy2 calculated from the CV at Õ s 10 mV sy1 using the equation of Q s nFA Gs . The value is fairly close to that for monolayer adsorption of 1 =10y10 mol cmy2 and again reveals the thin layer behavior of the Tosflexr FeŽCN. 63y film. This is comparable to that of previous report on the Nafion-RuO2-RuŽbpy. 32q composite electrodes w18x. Therefore, it is assumed that the AA oxidation reaction may occur throughout the whole Tosflexr FeŽCN. 63y film. Tosflex, an analogue of Nafion, contains different interfacial zones in the internal structure and thus leads to a porous nature. The partition coefficient between solution phaserpolymer film can thus be taken as unity since the analyte can diffuse relatively fast in the films. Note that

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J.-M. Zen et al.r Electrochemistry Communications 2 (2000) 782–785

Fig. 3. A plot of i pa versus wAAx on the FDTME. Other conditions as in Fig. 1. Non-linear curve fitting was performed using hyperbolic function. Insert figure shows the calibration plot under hydrodynamic condition in pH 5 PBS.

Fig. 2. The pH effect to D Ep ŽA. and Epa ŽB. at the FDTME in pH 5 PBS. ŽC. The pH effect to i pa with the addition of 1 mM AA. Scan rate is 50 mV sy1 and 100 mV sy1 for ŽA. and ŽB.&ŽC., respectively.

similar assumption was already made by Lyons et al. for the organic oxidation reaction on NafionrRuO2 composite carbon paste electrodes w19x. Under this condition, the three-dimensional effect of participation ŽAndrieux and Saveant model w20,21x. from the TosflexrFeŽCN. 63y may be restricted to two-dimensional behavior. The Andrieux and Saveant model can thus be deduced to the simple surface saturation kinetics. Fig. 3 shows the experimental and theoretically fitted results for the oxidation of 0–50 mM AA at the FDTME in pH 5 PBS. The oxidation current increases rapidly at lower concentrations and at-

tains asymptotic behavior at high concentrations. This is the mechanistic indication of an electrochemical reaction occurring with the formation of an intermediate complex between the mediator and substrate followed by its decomposition. In other words, the surface saturation kinetics in terms of Michealis-Menten ŽMM. mechanism w19,22x is operating in the present case. The theoretically fitted curve was obtained from non-linear least square regression program based on Marquardt-Levenberg algorithm for the MM equation as follows w19,22x: i pa s nFAk c Gs w AA x r Ž K m q w AA x . where the k c is the first order chemical rate constant, K m is the MM rate constant and other symbols have their own significance. The heterogeneous rate constant value kXe can then be obtained by using the equation of kXe s k c GsrK m . The reaction path for AA oxidation on the FDTME by the MM mechanism can be shown as follows:

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4. Conclusions The FDTME shows effective electrocatalytic oxidation of AA through the redox mediation of FeŽCN. 63yr4y. The FDTME also has stable responses over a much wider pH range than the FDPME. The mediated electrocatalytic oxidation indicates a surface saturation mechanism and was explained by the Michaelis-Menten kinetics. The amperometric determination of AA was demonstrated using i–t curves under hydrodynamic conditions and the obtained results were very sensitive. Acknowledgements The authors gratefully acknowledge financial supports from the National Science Council of Republic of China. References

Fig. 4. Comparative CV graphs for the FDTME and FDPME without ŽA. and with ŽB. 1 mM AA in pH 5 PBS at a scan rate of 100 mV sy1 .

The MM kinetic parameters evaluated are: k c s 8.90 sy1 , K m s 0.32 mmol dmy3 and kXe s 2.5 = 10y3 cm sy1 . These values are quite comparable to earlier MM kinetic values for system with mediation mechanism w19x. Fig. 4 shows comparative cyclic voltammograms for 1 mM AA on the FDTME and FDPME in pH 5 PBS. Obviously, the FeŽCN. 63y anion is not effectively exchanged into the PVP polymer. This is as expected since the working pH range of PVP is limited to more acidic solutions typically below pH 3 w23x. This phenomenon then resulted in the inactivation of the FDPME towards AA oxidation in pH 5 PBS as shown in Fig. 4B. This is indeed an advantage of the FDTME, which can be used at any pH to accumulated anion mediators. Quantitative amperometric response of the FDTME towards AA detection was studied using i–t curve under hydrodynamic condition in pH 5 PBS at an applied potential of 300 mV versus AgrAgCl. The measured amperometric current responses show a linear range up to 50 mM Ži.e., 8.81 mg ly1 . with a slope and intercept of 26 mA mMy1 and 0.12 mA, respectively. Standard deviation for 10 repetitive determinations was found to be 100 " 0.5. Note that the slope and the detection range is ; 20 times higher than a recent report for AA detection based on electrocatalytic oxidation using a rutheniumŽIII. diphenyldithiocarbamate-modified carbon paste electrode w24x.

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